This invention relates to a cascade-connected transistor amplifier and, more particularly, to a cascade-connected amplifier formed of a specific type of bipolar transistor which is connected in cascade to a field effect transistor (FET) having triode-type dynamic characteristics.
Cascaded amplifiers find ready utilization in diverse applications, especially wherein high gain of an amplified signal is desired. If each cascaded stage provides a signal gain, then the total amplifier gain is a function of the cumulative gains of the respective stages
Cascaded amplifiers serve additional functions, and one such function is signal equalization. Equalization is used in various signal processes and usually is provided to modify an initial signal so as to avoid deterioration thereof attributed to the inherent characteristics of a signal processing device. A compensating equalizing operation usually is necessary to restore the signal recovered from the signal processing device to its initial form.
In the field of recording signals on a phonograph record, it is conventional to equalize the input signals prior to recording in accordance with the so-called RIAA equalizing characteristics. These characteristics can be graphically represented as a family of frequency-related curves having two steps therein. Of course, if the recorded signal is equalized, it is preferable to compensate for such equalization when the recorded signal is reproduced from the phonograph record. This equalization can be accomplished by an equalizer amplifier formed of two or more stages of bipolar transistors. It is necessary for such an equalizer amplifier to have a high signal-to-noise ratio (S/N), to have a high signal gain and to provide minimal signal distortion. It is believed that an appropriate bipolar transistor for satisfying all of these prerequisites so as to be effectively used in an equalizer amplifier has not, heretofore, been developed.
Furthermore, when an equalizer amplifier must compensate for prior equalization in accordance with the RIAA characteristics, it is proposed to use an AC negative feedback circuit to provide such equalization compensation. However, such compensation must be provided over a relatively wide input signal frequency range and up to relatively high frequency signals. This would result in a low output impedance for such AC feedback circuit. Such a low output impedance would have an undesired effect upon the equalizer amplifier. Unfortunately, if the output impedance of the feedback circuit is high, the necessary compensation for the higher frequency signals cannot be attained.